Since the announcement of the first commercial installations of Octafining in Japan in June, 1958, this process has been widely installed for the supply of para-xylene. See "Advances in Petroleum Chemistry and Refining", volume 4, page 433 (interscience Publishers, New York 1961). That demand for para-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Para-xylene is a valuable chemical feedstock which may be separated for use in the synthesis of polyesters from mixed xylenes by fractional crystallization, among other methods. Benzene is a highly valuable product for use as a chemical raw material. Toluene is also a valuable product for use as a solvent, in chemical manufacturing processes, and as a high octane gasoline component.
Typically, para-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point .degree.F. Point .degree.F. Gal. ______________________________________ Ethylbenzene -139.0 277.1 7.26 Para-xylene 55.8 281.3 7.21 Meta-xylene -53.3 281.8 7.23 Ortho-xylene -13.8 291.2 7.37 ______________________________________
Calculated thermodynamic equilibria for the C.sub.8 aromatic isomers at 850.degree. F. are:
______________________________________ Wt. % Ethylbenzene 8.5 Wt. % Para-xylene 22.5 Wt. % Meta-xylene 48.0 Wt. % Ortho-xylene 21.5 TOTAL 100.0 ______________________________________
Principal sources of the mixtures of C.sub.8 aromatics are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range of 10 to 32 wt. % ethylbenzene (EB) with the balance, xylenes, being divided approximately 50 wt. % meta and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and it is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption (e.g., the Parex process), or membrane separation.
As commercial use of para-xylene and ortho-xylene has increased, isomerization of the other C.sub.8 aromatics to produce an equilibrium mixture of xylenes, and thus increase the yields of the desired xylenes, has become increasingly important. Octafining is one of the processes which produce an increased amount of xylenes.
In a typical plant utilizing the Octafining process, a mixture of C.sub.8 aromatics is introduced to an ethylbenzene tower wherein the stream is stripped of a portion of its ethylbenzene content, to an extent consistent with retaining all of the xylenes in the feed stream without unduly expensive "superfractionation." Ethylbenzene is taken off as overhead, while a bottom stream, consisting principally of xylenes, together with a significant amount of ethylbenzene, passes to a xylene splitter column. The bottoms stream from the xylene splitter, comprising primarily ortho-xylene and C.sub.9 aromatic, passes to the ortho-xylene tower from which ortho-xylene is taken off overhead, and heavy ends are removed. The overhead from the xylene splitter column is transferred to conventional crystallization separation. The crystallizer operates in the manner described in U.S. Pat. No. 3,662,013, incorporated by reference herein.
Because its melting point is much higher than that of the other C.sub.8 aromatics, para-xylene is readily separated in the crystallizer after refrigeration of the stream, and a xylene mixture lean in para-xylene is transferred to an isomerization unit. The isomerization charge passes through a heater, is admixed with hydrogen, and the mixture is introduced to the isomerizer.
Isomerized product from the isomerizer is cooled and passed to a high pressure separator from which separated hydrogen can be recycled in the process. The liquid product of the isomerization passes to a stripper from which light ends are passed overhead. The remaining liquid product comprising primarily C.sub.8.sup.+ hydrocarbons is recycled in the system to the inlet of the xylene splitter.
It will be seen that the system is adapted to produce quantities of para-xylene from a mixed C.sub.8 aromatic feed containing all of the xylene isomers plus ethylbenzene. The key to efficient operation to accomplish that result is the use of the isomerizer which takes crystallizer effluent lean in para-xylene and converts the other xylene isomers in part to para-xylene for further recovery at the crystallizer.
Among the xylene isomerization processes available in the art, Octafining was originally unique in its ability to convert ethylbenzene. Other xylene isomerization processes have required extremely expensive fractionation to separate ethylbenzene from other C.sub.8 aromatic fractions. As will be seen in the table of properties above, the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene. Complete removal of ethylbenzene from the charge by conventional methods, e.g., distillation, is therefore impractical. The usual expedient for coping with the problem is an ethylbenzene separation column in the isomerizer-separator loop when using catalysts other than those used in Octafining. However, Octafining does not require this expensive auxiliary equipment to prevent build up of ethylbenzene in the loop. This advantageous feature is possible because the Octafining catalyst converts ethylbenzene to xylenes.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes, which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethylbenzene to ethane and benzene, and hydrocracking of alkyl cyclohexanes.
A significant improvement over the Octafining process arose with the introduction of zeolite catalysts, such as zeolite ZSM-5, combined with a metal, such as platinum, as described in U.S. Pat. No. 3,856,872. At temperatures of about 700.degree. F.-800.degree. F., ethylbenzene is converted by disproportionation over the ZSM-5 catalyst to benzene and diethylbenzene. At higher temperatures, and in the presence of ZSM-5 catalyst of reduced activity, ethylbenzene and other single ring aromatics are converted by splitting off side chains of two or more carbon atoms as described in U.S. Pat. No. 4,188,282.
These developments permit upgrading of Octafining reactors by the substitution of the improved (ZSM-5) catalyst.
In many processes for xylene isomerization, conversion of ethylbenzene is constrained by the need to hold conversion of xylenes to other compounds to acceptable levels. Thus, although the above described advances provide significant improvements over Octafining in this respect, operating conditions are still selected to balance the advantages of xylene loss by disproportionation and the like.
A further advance in the art is described in U.S. Pat. No. 4,163,028, incorporated by reference herein, which is directed to xylene isomerization and ethylbenzene conversion at high temperatures with a ZSM-5 zeolite of very high silica/alumina ratio, whereby the acid activity of the catalyst is reduced. Other patents also disclose the use of ZSM-5 zeolite catalysts with reduced acid activity for high temperature (800.degree. F.) isomerization.
The inventions of those patents are predicated on discovery of combinations of catalyst and operating conditions which decouple ethylbenzene conversion from xylene loss in a xylene isomerization reaction, thus permitting the use of C.sub.8 fractions which contain ethylbenzene as the feed, without sacrifice of xylenes at conditions which will promote adequate conversion of ethylbenzene. These results are obtained by the use of a catalyst characterized by ZSM-5 zeolite substantially reduced in activity, e.g., by dilution, steaming, very high silica/alumina ratio, base exchange with alkali metal, coking or the like. At the high temperatures of 800.degree. F.-1000.degree. F., the reduced activity zeolite exhibits effective power for isomerization of xylene and for splitting off alkyl side chains of two or more carbon atoms from single ring aromatics at long on-stream periods. The disproportionation activity of the zeolite is severely depressed by the reduced acid activity, resulting in low losses of xylene by that mechanism. That lack of disproportionation activity impairs the capacity of the catalyst to handle trialkyl aromatics of nine or more carbon atoms, e.g., trimethylbenzene, as practiced in some processes. It thus becomes necessary to remove from the recycle stream those components having more than eight carbon atoms to avoid excessive build-up in the system of C.sub.9 and higher hydrocarbons. The catalyst also has the ability to crack paraffins in the charge to lower boiling compounds readily removable from recycle streams by fractionators normally present in the para-xylene recovery/isomerizer loop.
By reason of this combination of activities, the catalyst may used in a system charging reformate without removal of paraffin hydrocarbons, as described in U.S. Pat. No. 4,211,836.
U.S. Pat. No. 4,159,282 and Re. 31,782 to Olson et al., incorporated by reference herein, describe a xylene isomerization process in which a specified crystalline aluminosilicate zeolite characterized by a crystal size of at least about 1 micron is employed as an isomerization catalyst. In a more specific embodiment, the reaction is carried out with a crystalline aluminosilicate catalyst having a bimodal crystal size distribution generally falling in two ranges, less than about 1 micron and greater than about 1 micron with the latter being in major proportion.
The catalysts of zeolite, plus a metal, such as platinum, discussed above, are of the type known as "dual function catalysts" characterized by the provision of catalyst sites of different functions, each of which separately performs its function, often one step for each type of site in a multi-step reaction sequence. Such catalysts and the sequential reaction sites are discussed and explained by P. B. Weisz, "Polyfunctional Heterogeneous Catalysis," Advances in Catalysis, 13, pp 137-190 (1962). Weisz describes some experiments in which the two types of sites are provided by separate entities, such as physical mixtures of particles each of which provides only one type of catalytic site. Isomerization of certain paraffins over physical mixtures of acidic silica-alumina and platinum on a carrier is specifically described.